There is a requirement to provide directivity in small inexpensive underwater acoustic sensor assemblies suitable for use in sonobuoys and towed arrays. In sonobuoys, such a device is required to provide target-bearing information; in towed arrays, the device is needed to provide a means for resolving the left-right ambiguity inherent in a line of omnidirectional sensors. One of the simplest directional hydrophones consists of a dipole hydrophone in combination with a monopole hydrophone. The dipole hydrophone senses a (horizontal) vector component of the acoustic field (velocity, acceleration or pressure gradient), and the monopole hydrophone senses a scalar component (pressure). The two signals are added, with appropriate phase and amplitude adjustment, to form right-facing and left-facing cardioid directivity patterns: EQU P(.theta.,.phi.)=P[1+sin(.theta.)sin(.phi.)] EQU P(.theta.,.phi.)=P[1-sin(.theta.)sin(.phi.)]
where .theta. is the angle from the vertical, .phi. is the azimuth angle, and P is a reference amplitude.
A crossed dipole sensor for underwater acoustics measurements can be realized using pressure gradient hydrophone arrays, or particle velocity sensors. The use of pressure gradient hydrophones, or arrays of such hydrophones, is based on the principle of obtaining the first order spatial derivative by taking the difference between the outputs of two closely spaced omnidirectional hydrophones. The effectiveness of such devices may, however, be unacceptable at lower frequencies due to channel imbalances in phase and amplitude. In addition thereto, the pressure gradient hydrophone may have to be of considerable size if operation at low frequencies is required.
The particle velocity sensor offers an alternative to the pressure gradient sensor and, although it provides reduced control over sensitivity, it eliminates the channel imbalance problem. The particle velocity sensor concept can be realized by mounting an accelerometer in a container (preferably one which is neutrally buoyant) having dimensions which are small compared to an acoustic wavelength and without resonances in the frequency band of interest. Satisfactory designs for the particle velocity sensor have been obtained using moving coil accelerometers and piezoelectric bender elements. However, the particle velocity sensor may be unacceptable at low frequencies if the sensitivity of the accelerometer is not high enough to overcome the self-generated noise problem.
In addition to the above, problems have been encountered in trying to devise sensors with sufficient sensitivity to overcome self-generated noise at the lowest frequency of interest and with sufficiently wide bandwidth to process signals at the highest frequency of interest. Some accelerometer designs which provide adequate sensitivity for low frequency operation can introduce a device resonance in the listening bandwidth. In particular, known bender element designs exhibit an in-band resonance which can be expected to introduce channel imbalance in phase and amplitude from sensor to sensor in the vicinity of this resonance. An in-band resonance is objectionable because the frequency response of the sensor must be accurately known to permit effective combination of the particle velocity sensor signals with the signal from an omnidirectional hydrophone. Furthermore, this channel imbalance can be expected to be troublesome if beamforming applications with a number of such sensors are specified. The moving coil accelerometer referred to above is inherently expensive and may also exhibit an in-band resonance.
Thus, there is a need for a simple inexpensive accelerometer with sufficient sensitivity for acceptable low frequency operation and with the device resonance above the frequency range of interest; the frequency range of interest for some applications may extend over nine octaves. The crossed dipole sensor embodied in the invention will have a differential output, as opposed to a single-ended output, and its electrical impendance will essentially be capacitive, but it does not matter which vector component of the sound field is detected, as this merely affects the phase and amplitude adjustment of the signals before they are added together.